Tight Temperature-Salinity Relationship in Central Water Revealed by Argo Data

Abstract

Using temperature and salinity profiles from the Argo repository, distributions of water volume on temperature-salinity plane (so-called volumetric T-S diagrams or TSV plots) are compiled for the upper 2,000-m layer of the Atlantic, Indian, and Pacific oceans. The study is focused on Central Waters (CWs) that originate at the surface in the Subtropical Convergence and occupy the permanent thermocline within subtropical gyres. The South and North Atlantic, South Indian, and western North Pacific CWs (SACW, NACW, SICW, and WNPCW, respectively) are shaped as steep-sided ridges or narrow strips of elevated volume on the T-S plane (tight T-S relationship). In contrast, the western South Pacific, eastern South Pacific, and eastern North Pacific CWs (WSPCW, ESPCW, and ENPCW, respectively) do not have a tight T-S relationship and appear as still elongated but relatively wide poorly structured low elevations on TSV plots. Central Waters with tight T-S relationship are bordered from the poleward side by strong eastward baroclinic currents. The tight T-S relationship is found only in a denser part of CWs that outcrops within the strong eastward baroclinic currents and at higher latitudes. We hypothesize that enhanced isopycnal stirring and further diapycnal mixing of thermohaline irregularities in the strong meandering eddy-producing eastward currents substantially contribute to tightening the T-S relationship in CWs. By searching for same T-S characteristics in the main thermocline and at the surface, it was confirmed that CWs originate at the surface in the Subtropical Convergence in early spring and late winter.